1. Field of the Invention
The invention relates to an automatic transmission provided between an engine and a drive wheel in vehicles such as motor vehicles, railway vehicles, etc.
2. Description of the Related Art
Many vehicles adopt planetary gear-type multi-speed transmissions suitable for selecting an appropriate speed from a plurality of predetermined transmission speed ratios or gear speeds. With regard to such planetary gear-type multi-speed transmissions, it is desirable that the construction be simple and small in size, and that a great range of transmission gear ratios and speeds be allowed, and that the selectable transmission gear ratios change in a geometric progression or a similar fashion.
Planetary gear-type multi-speed transmissions capable of shifting speeds in seven steps have been proposed. Examples of such transmissions are planetary gear-type multi-speed transmissions in the third embodiment (FIGS. 5 and 6) and the fourth embodiment (FIGS. 7 and 8) in Japanese Patent Application Laid-Open No. 2000-266138. These transmissions allow great ranges of transmission gear ratios and speeds. However, since the transmissions substantially adopt four sets of planetary gear units, the transmissions have drawbacks of increased lengths and therefore degraded vehicle installability.